I was comparing small planes to an autonomous car, not the large airliners (maybe when Tesla makes giant buses, then yet, we can compare those). A 2+ ton Tesla into a crowd would do more damage than a similar sized, but lighter 2 or 4 seater Cessna plane crashing. Also, plane coming down in a crash would give people a bit more warning as most don't expect the plane to fly near people on streets, but a Tesla on a street is expected near until it gets up on the sidewalk. Lastly, with self driving cars and nobody in the car, the car's safety doesn't help the people or cars it hits. Airplanes don't travel only feet away from other airplanes and pedestrians - they have miles of exclusion zones around them for most of their travel, something you cannot enforce for autonomous cars.
And yes, even a single seater airplane is subject to strict FAA maintenance regulation.